This invention relates generally to micro-machined fluidic devices, including micro-fluidic apparatus, flow sensors, flow tubes, fluidic density sensors, flow controllers, chemical and biochemical systems, and the like, and, in particular, to the fabrication and implementation of flow measurement devices, fluid density measurement devices and fluidic circuits using silicon microfabrication and precision micromachining techniques.
Fluid measurement, control and manipulation are very important in many applications. Direct, accurate on-line measurement of mass flow and fluid density has been made possible through the development of different types of direct mass flow meters. One simple but effective device of this kind is known as the gyroscopic mass flow meter, which takes advantage of the Coriolis force in making measurements.
Since there is but one way of generating Coriolis forces, all existing devices based on gyroscopic or Coriolis force utilize the same basic principles, but specify different means for measuring the force. A number of approaches have been taken in utilizing Coriolis forces to measure mass flow. For instance, Roth, U.S. Pat. Nos. 2,865,201, 3,276,257, and 3,312,512, disclose gyroscopic flow meters employing a full loop, which is continuously rotated (DC type) or oscillated (AC type). The first commercial Coriolis mass flowmeter was introduced in 1977 by Micro Motion Inc. (Boulder, Colo.), a member of Rosemount Instrumentation and Control Group. U.S. Pat. No. 4,109,524 teaches the basic principle of this sensor and its construction. Such direct mass flow meters were the first to provide direct, accurate, on-line measurement of mass flow and fluid density. Their major advantages are direct mass flow and fluid density measurements, good accuracy, and high stability. Their major shortcomings are large size and high cost.
Developments in microfabrication techniques and silicon micromachining technology have made it possible to make precision structures for fluidic applications. Silicon as a micromechanical material has been discussed in many papers (J. B. Agnell et. al., 1983, K. E. Bean, 1978, K. E. Petersen, 1982). J. Chen and K. D. Wise have described the methods of making micromachined tubes and channels in silicon. In addition, Peter Enokssen et al. has reported bulk micromachined resonant silicon tube density sensors and mass flow sensors using optical techniques. The need remains, however, to exploit silicon microfabrication techniques to a greater extent in fabricating fluidic apparatus, including direct mass flow meters. It would be particularly advantageous to apply silicon microtubes and other technology to the development of gyroscopic mass flow meters based upon the Coriolis effect.
This invention broadly relates to micromachined fluidic apparatus. Generally peaking, such apparatus comprises a micromachined tube on or within a substrate, herein a portion of the length of the tube is free-standing relative to a surface of the substrate, enabling at least the free-standing section to move, twist, vibrate or otherwise deform. One or more electrodes associated with the free-standing section of the tube, in conjunction with one or more electrodes on an opposed, facing portion of the substrate, are used to actuate or control the movement of the free-standing tube section relative to the substrate, or to sense the movement of the free-standing tube section, or both. Electronic circuitry, which may be disposed on the same substrate as the fluidic circuit, is operated with respect to a variety of applications, including fluid flow measurement, fluid density measurement, fluid viscosity measurement, as well as fluid transport, separation and mixing.
Although various techniques may be used to actuate and sense tube movements, capacitive or electrostatic actuation techniques are used to control or resonate the tube, and to detect variations in tube movement for different applications. The capacitive technique is preferred, since the electrodes may be fabricated in the form of capacitor plates integrated to the overall apparatus, with one plate being disposed on the micromachined free-standing tube section, and the other plate being disposed in facing relation on the substrate. According to a specific preferred embodiment, the free-standing section of the tube is resonated for fluid flow and density measurements. A first set of electrodes are used to actuate tube vibration, and adjoining sets of electrodes are used on either side of the actuation electrodes in the corners of a U-shaped tube to facilitate measurements based upon the Coriolis affect, which are described in detail.
Three preferred methods of fabricating micromachined fluidic apparatus for the detection of fluid flow, density and viscosity are also disclosed. The first preferred method utilizes selective etching, boron-doped silicon and silicon fusing bonding to achieve a free-standing micromachined tube section on an insulating substrate. A second preferred method utilizes a buried silicon dioxide layer or doped silicon layers, to create etch stops, with high aspect ratio etching techniques and silicon fusing bonding being used to create a free-standing micromachined tube system, also on a non-conducting substrate. The third preferred method of fabrication takes advantage of the anisotropic etching of silicon, and selective etching, using boron-doped silicon and refill techniques. According to this embodiment, the microtubes are created with respect to a silicon substrate using a boron-doped top layer having chevron-shaped slit-like openings, with anisotropic etching of the underlying undoped silicon being used, followed by boron doping of the tube side-walls and bottom. As a final step, the chevron-shaped openings are refilled to create sealed microtubes.